Subsurface drilling has application for example in recovering petrochemicals from subsurface reservoirs. In a typical drilling operation a rotating drill bit at the end of a drill string cuts a borehole. The drill bit may be rotated by turning the entire drill string and/or using a downhole motor such as a mud motor. Cuttings released by the drilling operation are generally removed from the borehole by flowing drilling fluid through the drill string. The drilling fluid flows back to the surface in an annular region of the borehole surrounding the drill string. Subsurface drilling may be used to make boreholes that are very deep (e.g. thousands of meters).
It is common to case a borehole after the borehole has been drilled. Casing typically takes the form of a steel pipe that surrounds the borehole and extends to a desired depth. Casing is used among other things to prevent fluids from entering or leaving the borehole and to preserve the integrity of the borehole. Casing may extend to very significant depths in the borehole.
It can be desirable to establish data communication between downhole equipment within a borehole and surface equipment. Such data communication may, for example be used to transmit measurements from downhole sensors. The measurements may, for example include one or more of: measurements of downhole conditions (e.g. temperature, pressure and/or vibration levels), well logging data (e.g. neutron, gamma, magnetic and/or resistivity measurements of formations surrounding the borehole), steering information such as direction and inclination of a part of the drill string, and information regarding the status of items of downhole equipment. This information may be used for a wide variety of purposes including controlling drilling operations, scientific inquiry, mapping downhole formations, etc.
One application of data telemetry is to carry measurement while drilling (MWD) information. MWD is often applied for directional drilling. A typical directional drilling assembly provides a downhole motor at a slight bend (typically 0-5 degrees) near the drill bit. The bend causes the drill to extend the wellbore in a direction that deviates from the current trajectory of the well bore when the drill pipe is held stationary at surface and drill bit rotation is driven entirely by the downhole motor. To drill in a desired direction, the operator or directional driller needs to understand the orientation of the deflecting mechanism. A device commonly known as a MWD tool is included in the BHA. The MWD tool provides the directional driller with realtime information about the orientation of the downhole motor and provides periodic measurements of the direction and inclination of the drilling tool in the wellbore. The MWD system encodes this information into a binary data stream that is transmitted to a surface computer that decodes the data stream and presents the information to the directional driller on surface or in a remote location via suitable data link.
Data telemetry from deep underground is technically challenging. A variety of technologies have been developed for downhole data telemetry. These different technologies each offer tradeoffs between factors such as cost, available data rate, compatibility with existing drilling equipment, and reliability. One technology that is advantageous in some applications is electromagnetic (EM) telemetry. EM telemetry transmits data by generating electrical signals that propagate through the earth.
A typical EM telemetry system for transmitting data from downhole equipment to surface equipment includes a drillstring component usually called a ‘gap sub’ that provides electrical insulation between parts of the drillstring above and below the gap sub. A downhole EM telemetry transmitter applies electrical potentials (voltages) across the gap sub. These electrical potentials are varied with time in a way that encodes data for transmission to the surface. EM telemetry signals typically have very low frequencies (e.g. frequencies of less than 20 Hz). Frequencies of 1 Hz or lower may be used in some cases. Higher frequencies tend to be attenuated too strongly to be detected at surface. Any of a wide variety of encoding schemes may be used to encode data into an EM telemetry signal.
Application of potential differences across the gap sub allows electrical current to be transmitted into the formations surrounding the drill string. This current is typically detected using an array of ground stakes (usually copper coated rods driven into the ground) at surface. Patterns of variation in the detected current are processed at surface equipment to recover the encoded data.
Detected signals typically have very low amplitudes (e.g. microvolt-level). Electrical noise from pumps and other electrical equipment associated with a drilling operation may interfere with reception of EM telemetry signals.
EM telemetry signals can be strongly attenuated by certain formation types such as salts, and anhydrites. Such formations present a physical barrier to transmission of EM telemetry signals to the surface. Even in the absence of such formations the attenuation of EM telemetry signals presents a limit to the depth from which EM telemetry signals can be successfully transmitted to surface equipment. Using water-based drilling fluids also tends to increase attenuation of EM telemetry signals.
EM telemetry signals are often very weak when they reach the surface (received signals are often measured below 100 μV). Electrical equipment associated with a drilling operation (e.g. electrical generators, top drives, draw works, etc.) often emits electrical currents into the surrounding formations that can interfere with successful reception of data transmitted by EM telemetry at the surface. It is not uncommon for multiple directional drilling operations to be conducted simultaneously in close proximity to one another with each drilling operation using a different frequency for EM telemetry transmissions to allow for differentiation of the signals detected at surface.
U.S. Pat. No. 8,400,326 describes instrumenting an appraisal well on the sea floor to receive data telemetry from another well for offshore applications. The instrumentation in the form of one or more telemetry receivers may be permanently installed in the appraisal well. Such permanently installed receivers may be prohibited in land-based drilling applications. It may be difficult or impossible to change the location of a permanently installed receiver in an appraisal well after installation. For example, it is not uncommon that the feedback from drilling operations in a region reveals information about the existence of highly conductive formations (i.e. formations that make EM transmission difficult) which might not have been detected in any prior resistivity analysis.
U.S. Pat. No. 9,459,371 describes a system for receiving EM telemetry signals that involves a retrievable cable antenna that can be lowered down another nearby borehole to receive EM telemetry signals. This system relies on the availability of a suitable nearby borehole in which the cable antenna can be deployed. This system would not be expected to work in a cased borehole because the electrically-conductive casing would act as a shield, preventing reception of EM telemetry signals at the cable antenna.
There remains a need for practical and cost effective ways to receive EM telemetry signals that improve on existing technologies.